Pressed parts are used in general structural components. The material of a pressed part is a metal sheet such as a steel sheet. A structural component is formed from a single pressed part or formed by joining a plurality of pressed parts. For example, the structural components for automobiles described in Japanese Patent Application Publication No. 2013-189173 (Patent Literature 1) and Japanese Patent Application Publication No. 2014-91462 (Patent Literature 2) each include a vertically-long pressed part. The cross section of the pressed part is U-shaped.
FIGS. 1A and 1B show an example of a structural component. Of these drawings, FIG. 1A is a perspective view of the structural component, and FIG. 1B is a cross-sectional view of an end portion of the structural component illustrated in FIG. 1A. The structural component 20 illustrated in FIGS. 1A and 1B includes two pressed parts 21, each having a U-shaped cross section. Each of the pressed parts 21 includes a plate portion 24 and flanges 22 extending from the both sides of the plate portion 24. By welding the flanges 22 of the two pressed parts 21 together, the structural component 20 in the shape of a square-pipe is obtained. Reinforcing plates 40 are welded to the back side of the two plate portion 24 and four ridge portions 23 of the structural component 20, at both end portions in the longitudinal direction. In this case, however, the strength of the structural component 20 is increased only at both end portions in the longitudinal direction. Therefore, it can be considered that the reinforcement of the structural component 20 is not sufficient.
In order to produce a partly-reinforced structural component such as the structural component 20 illustrated in FIGS. 1A and 1B, it is necessary to weld the reinforcing plates 40 to the portions that need to be reinforced. Accordingly, a welding process must be separately carried out to partly reinforce the structural component 20, which results in an increase in manufacturing cost.